Steel concrete bond potentials in self-compacting concrete ... · been successfully implemented in both VC and SCC mixes. Pozzolanic activity, cementitious properties, smooth surface

Advances in Concrete Construction, Vol. 1, No. 4 (2013) 273-288

DOI: http://dx.doi.org/10.12989/acc2013.1.4.273 273

Copyright © 2013 Techno-Press, Ltd.

http://www.techno-press.org/?journal=acc&subpage=7 ISSN:2287-5301(Print)2287-531X(Online)

Steel – concrete bond potentials in self-compacting concrete mixes incorporating dolomite powder

Mounir M. Kamal1, Mohamed A. Safan1

and Mohamed A. Al-Gazzar2

1Department of Civil Engineering, Faculty of Engineering, Menoufia University, Egypt

2Water Research Center, Ministry of Irrigation, Al-Kanater, Egypt

(Received January 20, 2012, Revised October 11, 2013, Accepted October 20, 2013)

Abstract. The main objective of this research was to evaluate the potentials of self-compacting concrete (SCC) mixes to develop bond strength. The investigated mixes incorporated relatively high contents of dolomite powder replacing Portland cement. Either silica fume or fly ash was used along with the dolomite powder in some mixes. Seven mixes were proportioned and cast without vibration in long beams with 10 mm and 16 mm steel dowels fixed vertically along the flowing path. The beams were then broken into discrete test specimens. A push-put configuration was adopted for conducting the bond test. The variation of the ultimate bond strength along the flowing path for the different mixes was evaluated. The steel-concrete bond adequacy was evaluated based on normalized bond strength. The results showed that the bond strength was reduced due to Portland cement replacement with dolomite powder. The addition of either silica fume or fly ash positively hindered further degradation as the dolomite powder content increased. However, all SCC mixes containing up to 30% dolomite powder still yielded bond strengths that were adequate for design purpose. The test results demonstrated inconsistent normalized bond strength in the case of the larger diameter compared to the smaller one.

Keywords: self-compacting concrete; bond; pull-out; push-out; filler; dolomite powder

1. Introduction

Self-compacting concrete (SCC) refers to a normal or high strength concrete that is flowable

enough to be placed without need for compaction due to the ability of filling the forms under

gravity. Despite the high flowability of SCC, the mixes are sufficiently stable to resist segregation.

Japanese researchers began developing the material in the 1980s to ensure high quality in concrete

construction (Ozawa et al. 1989). The availability of this type of concrete provided unique merits

including faster construction rates since the concrete places so quickly, reduced labor since no

vibration is needed, the quality of compaction is no longer relevant, good finished surface quality

and design flexibility in detailing congested steel reinforcement with regard to the limited size of

coarse aggregates and the passing ability of SCC (Chan et al. 2010). Compared to conventional

vibrated concrete (VC), SCC mixes typically use higher contents of fine materials and less coarse

aggregate content. The maximum nominal size of the coarse aggregate is typically no more than

19 mm. Also, the use of a high range water reducer is a must to make the mix flowable at a

Corresponding author, Professor, E-mail: [email protected]

Mounir M. Kamal, Mohamed A. Safan, Mohamed A. Al-Gazzar

reasonable water-to-cement ratio. Viscosity agents are sometimes used to make the mix robust and

less sensitive to accidental variations in the proportions and/or quality of the used materials.

Increasing the content of fine materials is typically achieved by the increased content of sand and

other by-product materials that have specific potentials for being implemented in concrete

manufacturing. Silica fume, fly ash, granulated blast furnace slag and limestone powders have

been successfully implemented in both VC and SCC mixes. Pozzolanic activity, cementitious

properties, smooth surface texture and fineness are appealing features that can be provided by

these materials to improve the workability, strength and durability (ACI 232.2R 2003, ACI 234R

2006 and Dunster 2009). In addition, the consumption of by-product materials has a positive

impact on the environment and can also reduce the energy consumption in cement factories.

The authors have conducted extensive experimental testing to explore the potentials of low-cost

SCC incorporating relatively high contents of dolomite powder. Early research was devoted to

explore the productions aspects, the rheological properties and short term strength for single,

binary and ternary systems of silica fume, fly ash and dolomite powders replacing Portland cement

(Kamal et al. 2008, Al-Gazzar 2009). Later, SCC mixes selected on compressive strength criterion

were used to study the flexure and shear strength of reinforced concrete beams (Safan 2011a).

Also, these mixes were used to prepare reinforced concrete beams that were subjected to severe

corrosive environment (Safan 2011b). The results obtained were very encouraging for

implementing binary systems of dolomite powder and either silica fume or fly ash to replace

Portland cement in SCC mixes.

In the authors’ previous works (Kamal et al. 2008, Al-Gazzar 2009, Safan 2011a), dolomite

powder (DP) rather limestone powder was used to replace Portland cement for two reasons. The

first was the availability of the fine DP in local plants for ready-mix asphaltic concrete and the

second was the restrictions of the Egyptian Code of Practice (ECP 203-2007) that did not allow the

use of Portland limestone- cements (PLC) in reinforced and prestressed concrete. While the code

did not give an explanation for this restriction, a survey on the subject showed that the consulted

international specifications were concerned about some durability considerations. For instant, the

French Cement Standards in 1979 allowed the use of PLC containing up to 20% limestone as low

cost cement for ordinary buildings not subject to severe exposure (Assié 2007). The British

standard BS 5328-1 (1997) restricted the use of PLC in reinforced concrete subjected to sever

chloride exposure or freezing and thawing conditions, while its use was allowed in class 1 sulfate

conditions (SO4 concentration in the soil is less than 400 mg/l). BS 8500 (2002) reported that the

BS 5328-1 durability restrictions were due to lack of information and stated, based on recent

available data, that the performance of concrete made with PLC types containing up to 20 percent

limestone is equivalent to some other permitted cement types. However, BS 8500 (2002) still

restricted the use of PLC in concrete exposed to sulfate bearing environment classified as DS-1

(SO4 concentration in the soil is less than 500 mg/l). Later, a revision of BS 8500 (2006) adopted

the changed guidance in BRE SD1 (BRE Special Digest 1- 2005) and permitted using PLC in DS-

2 sulfate conditions (SO4 concentration in the soil 500-1500 mg/l). Currently, BS EN 197-1 allows

CEM II Portland-composite cement to contain up to 35% limestone replacement levels. However,

only CEM II/A-L and CEM II/A-LL containing 6-20% limestone are allowed for use in DS-2

sulfate conditions according to BS 8500 (2006). Recently in 2009, the Canadian Standards

Association (CSA) standard CSA A23.1 (2009) permitted the use of PLC containing up to 15%

limestone in all classes of concrete except sulfate- exposure classes. Hotton et al. (2007) and

Thomas et al. (2010) reported that PLCs were the largest single type of cement currently used in

the European countries in a variety of works including pavement works and precast concrete. They

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Steel – concrete bond potentials in self-compacting concrete mixes

reported that the benefits of PLCs include: reduced greenhouse gas emissions due to reduced CO2

as the clinker factor in the cement is reduced, improved workability and pumpability, similar

physical performance to current cements when the cements are properly optimized and similar

durability to chloride ingress and alkali silica reaction.

The current work was devoted to study the bond strength between steel bars and SCC mixes

containing different combinations of fillers. Steel-concrete bond is a fundamental property to

ensure the integrity of reinforced concrete structural elements because efficient bond ensures

reliable force transfer between reinforcement and the surrounding concrete. In case of a deformed

steel bar, the following mechanisms contribute to force transfer: (1) Chemical adhesion between

the bar and the concrete, (2) Frictional forces arising from the roughness of the interface and (3)

Mechanical anchorage or bearing of the ribs against the concrete surface. It has been demonstrated

that the bond strength is governed by the mechanical properties of the concrete, the volume of the

concrete around the bars in terms of the concrete cover and bar spacing parameters; the presence

of confinement in the form of transverse reinforcement, which can delay and control crack

propagation; and the diameter and geometry of the bar (deformation height, spacing and face

angle) (ACI 408R 2003, Eligehausen et al. 1983, FIB 2000, Lutz and Gergely 1967 and Tepfers,

1979).

There were only limited researches on bond strength in SCC providing conclusions that were

not comparable. In the bond tests carried out using pull-out specimens, De Almeida et al. (2005)

obtained equal bond strengths with SCC and VC, Chan et al. (2003), Daoud et al. (2003) obtained

5% higher strengths with SCC, while Zhu et al. (2004) reported strengths that were up to 25%

higher with SCC, with lower bond stresses as the bar size increased. Turk et al. (2010) studied the

effect of using different types and contents of fly ash and silica fume on the bond strength using

splice specimens in different SCC and VC mixes. The results showed that SCC demonstrated

higher bond strengths compared to VC due to superior filling and covering capability. Moreover,

the beam specimens produced from SCC containing 5% replacement of silica fume had the

greatest stiffness as a result of the pore structure improvement due to the pozzolanic activity of

silica fume.

2. Steel – concrete bond test specimens

A variety of test specimen configurations have been used to study bond between reinforcing

bars and concrete. The details of the specimen affect both the measured bond strength as well as

the nature of the bond response. According to ACI 408R (2003), the four most common

configurations are the pull-out specimen, beam end specimen, beam anchorage specimen and

splice specimen. Pull-out specimens are widely used because of ease of fabrication. However, they

are considered to be less realistic because the stress fields within the specimen simulate few cases

in actual construction. As the bar is placed in tension, the concrete is placed in compression.

Further, compressive struts form between the support points for the concrete and the surface of the

reinforcing bar, placing the bar surface in compression. To the authors’ knowledge, the literature is

lacking real explanation concerning the effect of such compressive struts on the bond strength.

More details about this test and its limits can be found in the FIB (2000). On the other hand, beam

anchorage and splice specimens represent larger-scale specimens designed to directly measure

development and splice strengths in full-size members. The anchorage specimen simulates a

member with a flexural crack and a known bonded length. The splice specimen, normally

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fabricated with the splice in a constant moment region, is easier to fabricate and produces similar

bond strengths to those obtained from the anchorage specimen.

3. Research significance

This research was a part of an extensive program to explore the potentials of low-cost self-

compacting concrete. Earlier, the production aspects, rheology and short-term compressive

strength were investigated for SCC mixes incorporating different combinations of by-product

fillers. Also, the shear strength in RC beams and the performance of RC beams in an aggressive

environment were investigated for selected mixes that demonstrated relatively superior strength

performance. The current work was devoted to explore the bond strength between steel bars and

SCC. The main parameters were the diameter of the steel bars, composition of the filler material

and variation of compressive strength along the flow path of SCC. The research presents

correlations between the experimental bond strength and compressive strength to asses the

capability of the different mixes to develop adequate levels of bond strength. Actually, the bond

strength is strongly influenced by the adhesion characteristics and the mechanical prosperities of

the surrounding concrete. The current research results are expected to provide useful information

concerning the bond strength as an essential prosperity for the integrity of structural elements with

regard to the variable composition of SCC among the other mentioned test parameters.

4. Materials

Cement and fillers: cement type CEM I 32.5 N meeting the requirements of BS EN 197-

1:2000 was used. The specific gravity of cement was 3.13 and the initial setting time was 90 min.

at 27.5 percent water for standard consistency. Locally produced densified silica fume was

delivered in 20-kg sacks. According to the manufacturer, the light-gray powder had a specific

gravity of 2.2, specific surface area of 17 m2/gm, loss on ignition of 1.5%, and 93 percent SiO2

content. Imported class F fly ash meeting the requirements of ASTM C618 was used. According to

the manufacturer, the average sum of SiO2, Al2O3 and Fe2O3 is 85 percent by weight with a

specific gravity of 2.1, and loss on ignition of 1.25 percent. The particle size distribution curve,

Fig. 1, shows that 90 percent by weight of ash passes through the 45-μm sieve. The dolomite

powder was obtained as a by-product from a local plant for ready-mix asphaltic concrete. The

production processes include drying the crushed dolomite used as a coarse aggregate by heating at

a degree of 120oC and sieving the aggregates to separate the different sizes. A small fraction of the

powder that passes through sieve No. 50 (300 μm) is used in the mix, while most of the powder is

a by-product. This powder had a light brownish color, specific gravity of 2.72. Sieving six random

samples of the powder showed that the average passing percentage through the 45-μm sieve was

63 percent. The chemical analysis results of the fine materials are reported in Table 1. The grading

of the used fine materials is shown n Fig. 1.

Aggregates: natural siliceous sand having a fineness modulus of 2.54 and a specific gravity of

2.65 was used. Crushed dolomite with a maximum nominal size of 16 mm was used as coarse

aggregate. The aggregate had a specific gravity of 2.65 and a crushing modulus of 23 percent.

About half of the particles were flaky and elongated. The grading of the used aggregates is shown

in Fig. 1.

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Admixtures: a traditional sulfonated naphthalene formaldehyde condensate HRWR admixture

conforming to ASTM C494 (types A and F) was used. The admixture is a brown liquid with a

specific gravity of 1.18.

Steel dowels: high tensile ribbed steel bars of 10 mm and 16 mm diameter were cut into 180

mm and 240 mm long dowels, respectively. The measurements reported in Table 2 and shown in

Fig. 2 describe the geometry of the rebars. The measurements were taken for the applied rebars

using a dial caliper having a precision of 0.05 mm.

Table 1 Chemical analysis of fine materials (% by mass)

Material CEMI 32.5-N Silica fume Fly ash (F) Dolomite powder

SiO2 24.3 93.2 49.0 0.83

Fe2O3 3.67 1.58 4.10 0.52

Al2O3 4.10 0.51 32.3 0.77

CaO 58.3 0.20 5.33 28.5

MgO 2.35 0.57 1.56 19.3

K2O 0.98 0.53 0.54 nd*

Na2O 0.35 0.45 0.28 nd

SO3 3.40 0.22 0.16 nd

CO2 nd nd 0.80 46.8

L.O.I 2.10 2.62 1.25 43.2

*nd: not detected

Table 2 Sieve analysis of fine aggregate

Nominal

diameter (mm)

Dimensions (mm)

d1 d2 w rs rw rh

10

16

9.9

15.9

10.4

17.3

2.0

1.7

4.2

6.9

2.2

3.1

0.75

1.25

Fig. 1 Particle size distribution for aggregates, dolomite powder and fly ash

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Fig. 2 Geometry of the steel bars

Table 3 Concrete mix materials and proportions

Mix

Mix Constituents *, kg/m

3

w/p cement sand dolomite water

dolomite

powder

silica

fume

fly

ash

M1 500 945 840 160 - -- -- 0.32

M2 450 935 830 165 50 -- -- 0.33

M3 400 925 820 170 100 -- -- 0.34

M4 350 915 810 170 100 50 -- 0.34

M5 300 900 800 175 150 50 -- 0.35

M6 350 915 810 170 100 -- 50 0.34

M7 300 900 800 175 150 -- 50 0.35

superplasticizer dosage in all mixes = 10 kg / m3 (2% by weight of powders, p)

Table 4 Mechanical and rheological properties of SSC mixes

Mix mechanical properties rheological properties

fcu (MPa) fr (MPa) fr / fcu0.5

slump flow (mm) V-funnel to (sec.)

M1 34.0 3.8 0.65 700 6.0

M2 33.0 4.0 0.69 665 6.4

M3 29.5 3.8 0.70 655 5.5

M4 35.0 4.1 0.69 600 5.5

M5 31.5 3.9 0.69 580 5.2

M6 30.5 4.0 0.72 650 5.0

M7 28.5 3.9 0.73 640 5.1

fcu: 28-day compressive strength , fr: modulus of rupture

4.1 Concrete mix proportions Based on the results reported in an initial phase of research (Kamal et al. 2008), seven mixes

were selected to produce SCC based on compressive strength criterion. The selected mixes

incorporated dolomite powder (DP) replacing up to 30% of cement weight along with either silica

fume (SF) or fly ash (FA) that replaced 10% of cement by weight.

The constituents of the selected SCC mixes are given in Table 3. In these mixes, the fine-to-

coarse aggregate ratio was 1.13, the total content of powders (cement and fillers) was 500 kg/m3,

the HRWR dosage was fixed at 10 kg/m3 (2% by weight of powders). The water content was

determined by trail and error procedure to obtain consistent mixes with the required fresh

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rheological properties. Table 4 shows the measured rheological properties and the mechanical

properties in terms of the compressive strength fcu and the modulus of rupture fr evaluated at 28

days.

4.2 Concrete mix proportions

Push-out test specimens were used in the current work. Generally, the weak points of push-out

test specimens, similar to pull-out test specimens, were the friction between the specimen and the

bearing plate, and the arch-effect in the region close to the bearing plate. For these reasons, the

bonded length was moved away from the bearing plate by providing a broken- bond zone next to

the bearing plate as can be seen in Fig. 3. The procedure adopted by Foroughi et al. to introduce a

broken-bond zone and to avoid an unplanned force transfer between the bar and the concrete in

this area was followed by encasing the bar with a plastic tube and sealing with a highly elastic

silicone material. Also, 10 mm broken-bond zone was provided at the loading end so that the

bonded length was five times the bar diameter in both 10 mm and 16 mm bar specimens.

Fig. 3 Configuration of push-out bond test specimens (a- 10 mm dowel inserted in 200 × 150

× 120 mm concrete prism & b- 10 mm dowel inserted in 200 × 150 × 180 mm concrete prism)

Fig. 4 Geometry and dimensions of the casting wooden forms

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4.3 Casting forms

Casting forms were needed to manufacture concrete beams by casting the SCC mixes at one

end and allowing concrete to flow to the other end without any compaction or vibration. The forms

were designed to make it possible to split each beam into seven bond specimens with a steel dowel

inserted at the center of each specimen. Specially designed wooden forms, Fig. 4, were

manufactured for this purpose.

The wooden forms had net internal dimensions of 120 mm depth, 150 mm width and 1400 mm

length for the 10 mm steel bar diameter specimens. The corresponding dimensions for the 16 mm

steel bar diameter specimens were 180 mm, 150 mm and 1400 mm. A separate plate was laid

along the bottom of the form. The bottom plate was provided with seven holes at 200 mm center-

to-center spacing to accommodate the lower end of the steel dowel. Vertical 25 × 25mm

polyurethane strips were cut and adhered along the depth of the two sides of the form at 200 mm

center-to-center spacing. The soft polyurethane strips were intended to provide notches along the

beam specimens to facilitate its splitting into discrete test specimens. After sampling the forms, a

thin layer of grease was applied to the internal faces to prevent water absorption and ensure easy

stripping of the forms. Upper horizontal plywood strips provided with holes were used to confine

the upper portion of the steel dowel and thus the dowels were held in a vertical position during

casting. Fig. 5(a) shows the wooden forms with the steel dowels inserted and confined. The width

of the concrete prism (150 mm) yielded concrete cover ratios (concrete cover to steel diameter) of

7 for 10 mm specimens and 4.2 for 16 mm diameter.

Fig. 5 Preparation and testing of push-out test specimens

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Two 40-liter batches were used to cast two beams of 120 mm and 180 mm depth along with

one 100 mm cube to determine the compressive strength, fcu, and one 100 × 100 × 500 mm prism

to determine the tensile strength in terms of the modulus of rupture, fr. Each concrete mix was used

to fill the forms and molds three times, so that each reported test result was the average of three

test results. After mixing, the concrete was poured in a wide vessel and successively hand-poured

in the forms and molds, Fig. 5(a). The forms were stripped 24-hours after casting. Fig. 5(b) shows

the beam specimens after stripping and sequentially numbering the bond test specimens. After that,

each beam was carefully poisoned in a hydraulic flexure machine to split it at the notched sections

and obtain the bond test specimens, Fig. 5(c). All specimens were cured under continuously wet

cloth that was moistened twice a day until testing at 28-day age. 4.4 Testing procedure

A total number of 147 bond test specimens, 21 cubes and 21 prisms were tested. Fig. 5(d)

shows the test set up and loading configuration of the push-out test. The bond test specimen was

tested under a compression force driving down the steel dowel. A 500kN universal testing machine

was used to apply the compression force at a loading rate of 50kN/min. The machine provided an

automatic control of the loading range to ensure precise load measurements. A 20 mm thick

bearing plate provided with a central hole was used to support the test specimen. The plate was

supported on the edges of a rigid base allowing the penetration of the dowel. A 3 mm packing

plywood plate was used to ensure even contact between the bottom surface of the concrete

specimen and the bearing plate. To prevent buckling of the 50 mm long upper free part of the

dowel under the applied load and to ensure eccentric loading, a special steel punched head was

fixed in the upper platen of the testing machine. The punched head confined 30 mm of the free

loaded part and thus a bar length of 20 mm was available for the dowel to penetrate through the

concrete block. The 20 mm maximum penetration value was more than sufficient to achieve the

ultimate bond strength knowing that the rip spacing was 4.2 mm in case of the 10 mm dowels and

6.9 mm in case of the 16 mm dowels. The test was ended once the ultimate load was recorded.

5. Analysis of test results

The tests results were indented to evaluate the bond strength potentials of SCC mixes

incorporating relatively higher dosages of dolomite powder replacement. Some measures, similar

to those adopted in pull-out tests, were adopted to overcome the deficiencies of the push-out test

specimen. The bonded length was shifted away from the bearing plate to avoid confinement effect

due to the lateral compression stress induced in the concrete. Also, a relatively limited bond length

of five times the steel bar size was adopted. These factors were expected to yield favorable bond

failures due to the slip of the bar rather than due to splitting of concrete. However, splitting failures

could still occur if the tensile strength is exhausted given that no radial steel reinforcement was

provided to resist splitting.

The test results were evaluated based on normalized bond strength obtained by dividing the

average bond strength of a given mix to the square root of the corresponding cylinder compressive

strength. The bond strength was calculated by dividing the ultimate load Pu by the bonded area

(.d.l). According to the ACI 408R (2003), it is both convenient and realistic for design purposes

to treat bond forces as if they were uniform over the bond length. Given that the bonded length (l)

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was five times the bar diameter (d) in the current work, the ultimate bond strength can be

expressed as

25 d

Pf u

b

(1)

Due to the variation of the concrete compressive strength, the bond strength was expressed in

terms of normalized bond strength fbn, where

cy

bbn

f

ff (2)

The cylinder compressive strength fcy was taken equal to 80% of the corresponding cube

compressive strength according to data collected by Domone (2007). To examine the adequacy of

the obtained levels of bond strength, two approaches were adopted. The first was to find how the

obtained normalized bond strength would compare to the lower limit or design value set by the

ACI code for this strength. The second was to compare the obtained results with the results

available in the literature for similar SCC mixes and bar diameters. For this purpose, the bond

strength was related to the square root of the cylinder compressive strength. The average bond

strength along the flowing path, the total average and the normalized bond strength are reported in

Tables 5-6.

The ACI 318R (2008) code did not provide an explicit expression linking the bond strength to

the concrete compressive strength. Rather, the bond strength was implicitly applied in the

calculation of the development length. The development length concept is based on the average

bond stress over the length of embedment of the reinforcement. For some favorable conditions

concerning the spacing between developed or spliced bars, clear cover and confinement, the

following equation is used by ACI 318R (2008) code to calculate the development length (l)

associated with relatively higher bond strengths for 19 mm and smaller steel bars

cy

y

f

dfl

1.2 (3)

Setting the ultimate force in the push-out bond test equal to fy Ab, then the bond strength (fb) can

be expressed as

dl

Aff

by

b

(4)

where: fy is the yield stress of steel and Ab is the area of the steel bar. Eq. (4) yields

b

y

f

dfl

4 (5)

The analogy between Eqs. (3)-(5) yields

cyb ff 53.0

(6)

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Eq. (6) shows that the normalized bond strength for design purposes is taken 0.53 in favorable

design conditions that require the development of satisfactory bond strength levels. The

normalized bond strength values in Tables 5-6 ranged from 1.21 to 1.93 for mixes (3-7) that had

the same failure mode. These values are 2.3 to 3.6 times the design value with an average of 2.4

and 2.8 for the 10 mm and 16 mm bars, respectively.

Fig. 6 shows a plot of the normalized bond strength obtained from pull-out tests against the

corresponding compressive strength (fcy). The data were collected for different SCC mixes that had

comparable compressive strength in the range of 26 to 36 MPa and steel bar diameters of 10, 14

and 16 mm (Domone 2007, Foroughi et al. 2008, Menezes et al. 2008). In this range, the

normalized bond strength varied from 1.07 to 2.6. It can be seen that the obtained normalized bond

strengths are within this range.

The 10 mm and 16 mm test specimens cast using mixes 1 and 2 failed due to splitting along the

shorter lateral dimension of 150 mm. The average normalized bond strength was higher in the 10

mm bar diameter specimens compared to the 16 mm diameter specimens. The decrease of the

bond strength for a given concrete strength as the steel bar diameter increased has been

Table 5 Bond strength (MPa) of the 10 mm bar specimens

Specimen serial

number

Mix No.

1* 2* 3 4 5 6 7

1 13.14 9.40 5.3 6.75 5.49 5.56 4.90

2 15.42 9.80 5.51 6.90 6.04 5.96 6.22

3 15.90 12.38 6.94 7.76 6.36 6.76 6.62

4 17.75 10.18 6.57 7.50 7.42 7.15 7.28

5 18.70 10.15 5.99 7.18 8.05 6.63 6.12

6 17.10 9.67 5.99 6.92 6.76 6.36 4.90

7 16.80 9.15 5.00 6.47 6.33 6.30 4.50

Average fb 16.40 10.10 5.90 7.07 6.64 6.39 5.80

fbn ( fb / fcy0.5

) 3.14 1.97 1.21 1.34 1.32 1.29 1.21

* test specimens failed due to splitting

Table 6 Bond strength (MPa) of the 16-mm bar specimens

Specimen serial

number

Mix No.

1* 2* 3 4 5 6 7

1 11.32 8.66 5.55 7.55 6.44 6.44 9.99

2 11.10 9.32 5.77 7.70 6.88 6.66 9.32

3 12.88 9.77 6.88 7.90 6.66 7.33 9.32

4 12.43 9.10 5.99 7.77 7.10 7.55 10.44

5 13.10 8.44 5.99 7.99 6.66 7.99 8.66

6 11.99 8.44 5.77 7.99 6.22 7.55 8.44

7 11.77 8.21 5.33 7.55 6.22 6.66 8.44

Average fb (MPa) 12.1 8.85 5.90 7.78 6.60 7.17 9.20

fbn ( fb / fcy0.5

) 2.32 1.72 1.21 1.47 1.31 1.45 1.93

* test specimens failed due to splitting

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Fig. 6 Normalized bond strength and corresponding compressive strength

Fig. 7 Bond strength along the flowing path for the 10-mm bar diameter specimens

Fig. 8 Bond strength along the flowing path for the 16-mm bar diameter specimens

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penetration of the steel par through the concrete without concrete cracking. This behavior can be

occasionally reported in the literature. The rest of the test specimens failed favorably due to the

attributed partially due to the relative improvement of the tensile strength expressed in terms of the

normalized values of the modulus of rupture as indicated in Table 4.

On the other hand, SCC mixes incorporating finely divided minerals were reported to

demonstrate improved interfacial transition zones between the paste and the steel bars. Zhu et al.

(2004) investigated the steel–concrete interfacial transition zone with a nano-indentation

technique, and found that the increased bond strength with the SCC mixes compared to vibrated

concrete could be attributed to a greater uniformity in the interfacial transition zone (ITZ) around

the bars. The relative improvement of the tensile strength and uniformity of the ITZ associated

with better adhesion can rationally explain the penetration of the steel bar extracting a uniform

layer of mortar between the ribs as observed after the test. This failure mode was associated with

two distinct observations. First, the reduction of the both the bond and normalized bond strength

due to cement replacement in all mixes. Second, the normalized bond strength was higher for the

16 mm bars in mixes 4, 6 and 7 compared to the 10 mm bars, while the normalized bond strength

was the same for the two diameters in mixes 3 and 5. The reduction of the bond strength in SCC

mixes seemed to increase as the percentage replacement with dolomite powder (DP) increased in

mix 3 compared to mix 2. Independent of the combination of the fillers in mixes (3-7), the average

normalized bond strength was 1.27 and 1.47 in case of the 10 and 16 mm bars respectively. Thus,

the average normalized bond strength was higher for the 16 mm bars by 16 percent. The addition

of either silica fume or fly ash seemed to have a positive effect even when the DP ratios increased.

The reduction of the bond strength due to the incorporation of the DP can be explained by

recalling that load transfer between concrete and steel occurs through the action of three

mechanisms: chemical adhesion, friction and mechanical interaction of the lugs of the deformed

reinforcement bearing on the surrounding concrete. For deformed reinforcement, mechanical

interaction is the dominant mechanism of response according to Zhu et al. (2004).

The use of the DP as cement replacement seemed to have a softening effect on the matrix,

while keeping an adequate chemical adhesion due to the improvement in the ITZ as mentioned

before. This behavior hindered the development of the mechanical interaction and the bond

strength was dominated by the friction resistance. Another parameter contributing to the reduction

of the bond strength is the reduced shrinkage of concrete upon cement replacement and

consequently the reduced gripping force exert by the concrete as reported by Sonebi et al. (1999).

The observed occasional increase in the bond strength for a bigger diameter was reported by Khan

et al. (2007). The increase was attributed to the increase in the friction bond component as the bar

diameter increased. Also, the inconsistency of bond strength results in SCC mixes has been

reported by Zhu (2001), Domone (2007) and it was recommended that each concrete mix with a

specified composition of the fillers should be tested to evaluate the bond behavior.

Figs. 7-8 show a plot of the average bond strength for the test specimens along the flow path

for the10 mm and 16 mm bars. The specimens were numbered sequentially beginning with the

first specimen at the casting end and ending with the seventh specimen at the end of the flow path.

It can be seen that the bond strength tended to be higher about the middle of the flow path

compared to the strength evaluated at the casting and end points. This trend needs to be further

examined in actual size concrete elements being related to size effect and flow characteristics.

Close observation of the flow and examining the homogeneity of fresh concrete, the compressive

strength, and the characteristics of the ITZ at different locations along the flow path are necessary

to have conclusive results.

285


6. Conclusions

The current work investigated the potentials of the SCC mixes incorporating dolomite powder

along with either silica fume or fly ash to develop bond strength. The variability of bond strength

along a flowing path was evaluated, based on the available test results; the following conclusions

could be drawn:

• The bond strength was reduced due to Portland cement replacement with dolomite powder.

While the chemical adhesion component was improved, the mechanical interaction was hindered

and the bond strength was dominated by the friction resistance.

• The addition of either silica fume or fly ash positively hindered further degradation of bond

strength as the dolomite powder content increased.

• SCC mixes containing up to 30% dolomite powder yielded bond strengths that were adequate

for design purpose according to the requirements of the ACI 318R-08 code.

• Independent of the combination of the dolomite powder, silica fume and fly ash as fillers in

the tested SCC mixes and for dolomite powder replacement ratios (20 and 30%), the average

normalized bond strength was higher for the 16 mm bars by 16 percent compared to the 10 mm

bars.

• The bond strength tended to increase about the middle of the flowing path. This trend needs to

be further examined in actual size concrete elements being related to size effect and flow

characteristics.

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Steel concrete bond potentials in self-compacting concrete ... · been successfully implemented in both VC and SCC mixes. Pozzolanic activity, cementitious properties, smooth surface